Since 1983, photomasks have been commercially available for deep cut sandblast engraving or sandcarving highly detailed single or multi-leveled engravings in glass, crystal, stone, and even certain metals, without costly die or template set-ups. This has been achievable by a utilizing a vendor remote from the use (sandblast/sandcarve) site to produce the photomasks. In the proprietary techniques for producing the photomask for such purposes, a silk screen emulsion (diazo sensitized polyvinyl acetate) is draw coated onto a silicone coated carrier sheet, which is dried in the dark. After drying, the coated sheet of emulsion is disposed with its emulsion face in contact with a film positive in a vacuum contact frame, and exposed to a 5,000 watt halogen UV light for 15 to 30 seconds. After exposure, the photomask is magnetically attached to a drum with the emulsion side out, inside a washout unit, and spun while 90 degree F. water at about 100 PSI is sprayed onto the drum and photomask, dissolving the unexposed (uncured) emulsion of the photomask in about three minutes. The photomask is then blotted and air dried for three hours. Finally, a thin coating of adhesive is applied with a Potdevin roller coater wet applicator machine, and a thin protective release sheet (silicone cover sheet) is placed over the adhesive.
While the photomasks produced according to the prior art process are of extraordinarily high quality and have met with great commercial success, the drawback associated with them is that they cannot be produced on the user site. That is the organization that will do the sandblast engraving or sandcarving does not have the facilities to produce photomasks utilizing this technique, nor would it be cost effective to obtain such equipment. Therefore there is a wait of two to seven days between the time of ordering of the photomasks from the supplier, and the time they are actually on the use site. This limits the potential market for personalized sandblast engraved or sandcarved items. Despite the fact that it has been known for many years in the art that the market for personalized sandblast engraved or sandcarved items would be substantially greater if the finished goods could be delivered in one to twenty-four hours, heretofore there has been no procedure or apparatus for practically accomplishing that result.
According to the present invention the major drawbacks associated with the prior art procedure which preclude its implementation at the photomask user's site are the expense of the vacuum contact frame, the energy requirements and practical utilization of the 5,000 watt UV light, the use of the spinning drum for washout of the uncured emulsion, the long drying time after washout, and the expense and sophistication of an adhesive applicator machine. All of these drawbacks are overcome by the method and apparatus according to the present invention. According to the invention it is possible to simply and relatively inexpensively, in a practical manner, effect the production of photomasks for sandblast engraving or sandcarving directly at a user's site.
According to the method of the present invention, a photomask having a carrier sheet covered by a cured design structure is produced from a stencil which comprises a carrier sheet (e.g. silicone coated paper carrier sheet) with a curable emulsion coating (e.g. silk screen emulsion). The method comprises the steps of substantially sequentially: (a) With a manual clamping structure, clamping a stencil in face-to-face contact with a print positive sheet having design elements, the stencil emulsion coating contacting the print positive. (b) Exposing the clamped stencil and print positive to emulsion curing radiation, to cure the emulsion except for areas covered by design elements of the print positive. (c) Separating the stencil from the print positive. (d) Directing streams of high pressure liquid at sufficient temperature, and with sufficient force, and for a sufficient period of time, against the stencil to remove uncured emulsion from the stencil. (e) Manually blotting the stencil. (f) Drying the stencil. And, (g) manually applying adhesive to the cured emulsion coating face of the stencil, to produce a final photomask.
By manual clamping of the stencil and print positive, the expense of a vacuum contact frame is eliminated. The manual clamping structure preferably used in the practice of the invention comprises a rigid material base having top and bottom faces with a compressible material covering on the top face of the base, and a transparent cover (typically heavy glass cover). Means are provided for pivotally mounting the transparent cover to the base so that the cover may pivot toward and away from essentially face-to-face contact with the compressible material (the stencil and print positive being sandwiched between the cover and the compressible material) .
Typically the step of exposing the clamped stencil and print positive to emulsion carrying radiation is practiced by utilizing an ultraviolet light mounted above the transparent cover, and controlled by a timer. The ultraviolet light is typically a lightbulb of less than 1,000 watts, e.g. a 500 watt halogen UV light source. While this takes a slightly longer curing time (e. g. 60 to 90 seconds compared to 15-30 in the sophisticated prior art method), it allows the utilization of a light source that is much smaller, thereby increasing safety and minimizing tile possibility that special power requirements, etc. will have to be utilized.
Step (d) is preferably practiced by mounting the stencil in a stationary position while directing moving streams of high pressure liquid onto the stencil. The use of a spinning drum in a washout unit is entirely impractical for use locations, therefore an apparatus is provided according to the invention which is a little more than a tank. The apparatus comprises a tank having open top portions, with a photomask supporting surface mounted within an area defined by the tank, the surface facing generally upwardly. A number of spray heads are mounted together above the surface for movement with each other, and means are provided for moving--typically reciprocating in a generally horizontal plane--the spray heads back and forth for directing liquid from them onto different portions of the surface as the spray heads are moved.
Preferably, a recirculating pump and a heater (which may be a combined unit) is mounted within the tank and connected by a conduit (at least a portion of which is flexible ) to the spray heads to heat and recirculate liquid from the tank to the spray heads. The heater heats the water to at least 90 degrees F., while the pump pressurizes water to at least about 100 psi . The supporting surface slants downwardly from one end to the other so that the water will rinse off the uncured emulsion, and the stencil may be held onto the supporting surface by magnets to the like.
Step (e) is preferably practiced by placing the stencil on a blotting towel and manually moving a blotting roller (e.g. a lamb's wool roller) over the stencil to remove water droplets. Drying the stencil must take place quickly. The air drying time of three hours, as in the commercial sophisticated method, is totally at odds with the physical structures and the results desired by use installations. According to the present invention, a microwave oven is provided which is utilized to effect drying. The stencil--after blotting--is placed inside the oven, and the microwave is activated for about eight minutes at its lowest ("defrost") power setting, which effects drying without adversely affecting the stencil.
Machine application of adhesive also is impractical for use sites. According to the invention, step (g) is accomplished by placing a pool of adhesive at one end of the stencil, after the stencil has been placed over a disposable backing (such as newspaper), and a draw bar is manually moved over the stencil to smooth the adhesive over the entire stencil. The liquid adhesive typically is a heat curable adhesive, although it may be a pressure sensitive adhesive for some applications, and is fully operational within five minutes. The excess adhesive that has been drawn onto the news print is discarded with the news print.
It is the primary object of the present invention to provide a simple, relatively inexpensive, and practical production of photomasks for engraving sandblasting or sandcarving at the engraving/sandcarving sites. This and other objects of the invention will become clear from an inspection of the detailed description of the invention, and from the appended claims.